TCP header options

Use TCP header options to check the properties of the TCP header.

src_port

Check the source port number or range.

Syntax:

--src_port [!]<number>;

The placement of : indicates less than or equal to:

--src_port [!]:<number>;

The placement of : indicates greater than or equal to:

--src_port [!]<number>:;

The placement of : indicates a range, exclusive of endpoints:

--src_port [!]<number>:<number>;

The optional prefix ! means exclude.

Example:

Greater than or equal to 1000

--src_port 1000:;

dst_port

Check the destination port number or range.

Syntax:

--dst_port [!]<number>;

Equal to:

--dst_port [!]:<number>;

Greater than or equal to:

--dst_port [!]<number>:;

Range, exclusive of endpoints:

--dst_port [!]<number>:<number>; placement of : indicates  a range, exclusive of endpoints

The optional prefix ! means exclude.

Example:

Greater than or equal to 100 and less than or equal to 200:

--dst_port 100:200;

seq

Check the TCP sequence number value or range.

Syntax:

--seq <number>[,relative];

Equal to:

--seq =,<number>[,relative]; 

Greater than:

--seq >,<number>[,relative]:; 

Less than:

--seq <,<number>[,relative]; 

Not equal to :

--seq !,<number>[,relative]; 

The optional field relative indicates the value is relative to the initial sequence number of the TCP session. No prefix defaults to "equal to."

Examples:

--seq <,12345;

--seq !,12345;

ack

Check the TCP acknowledge number for a specific value.

Syntax:

--ack <number>;

Equal to:

--ack =,<number>[,relative]; 

Greater than:

--ack >,<number>[,relative]:;

Less than:

--ack <,<number>[,relative]; 

Not equal to :

--ack !,<number>[,relative]; 

Examples:

--ack <,12345;

--ack !,12345;

tcp_flags

Specify the TCP flags to match in a TCP packet.

Syntax:

--tcp_flags <!*+FSRPAU120>[,<FSRPAU120>];

Flag	Description	Note
S	SYN	upper case required
A	ACK	upper case required
F	FIN	upper case required
R	RST	upper case required
U	URG	upper case required
P	PSH	upper case required
1	reserved bit 1
2	reserved bit 2
0	No TCP flags set	No TCP flags set

The first part defines the bits to match:

• The flags S, A, F, R, U, and P must be in upper case.
• If the first digit is 0, it will stop and ignore all of the following flags.
• * matches any one of the specified bits.
• + matches all of the specified bits, plus any others.
• ! matches if none of the specified bits is set.
• Default matches the specified bits exactly.

The second part is optional. It identifies the bits that should be masked off before matching.

Examples:

--tcp_flags 0,12;

--tcp_flags !SAFRUP,12;

 --tcp_flags S,12;

--tcp_flags S+;

--tcp_flags *SAFRUP12;

window_size

Check for the specified TCP window size.

Syntax:

--window_size [!]<number>;

--window_size [!] 0x<number>;

--window_size [>]<number>;

--window_size [<]<number>;

Examples:

--window_size 1000;

--window_size !0x1000;

tcp.src_port, tcp.dst_port, tcp.seq, tcp.ack, tcp.flags, tcp.window_size, tcp.checksum, tcp.urgent, tcp.any_option, tcp.payload_length

Check for these fields in the TCP header.

Syntax:

--tcp.[decorations] <operator><value>;

Valid operators: =, !, >=, <=, &, |, ^, and in.

Examples:

--tcp.src_port in [1111,2222];

--tcp.flags & 0x0f = 0x6;

Iterate over all options:

--tcp.any_option = 0x6052, dword;

tcp [offset]

Access any fields in TCP header in freelance mode.

Syntax:

--tcp[offset] <operator><value> [, word size] [, endianness];

Both word size and endianness are optional. By default, the engine uses BYTE and big endian.

Valid operators: =, !=, >=, <=, &, |, ^, and in.

Example:

--tcp[20] &0xF0 = 0x30;